Tried just manually adjusting the LUM only qualifier to include the bottom 10% of luminance, but I found I had to crank 'Low' all the way up to 50% to begin to get a matte...

Then, curious, I tried to qualify the brights and found I have to crank 'High' down to 75% to start to get a matte...

So now, to put a little science on it, I loaded up a RAMP. Sure enough, the darks don't start to be included in the matte until 'Low' get above 50%, and the brights don't start to be included in the matte until 'High' gets below 75%...

Resolve operates in 32bit float. That means you can have data below black and above white. This is handy for say recovering information lost in highlights or shadows, the qualifiers respond in correspondence with the data resolve is able to differentiate.